U.S. patent number 8,358,741 [Application Number 12/857,676] was granted by the patent office on 2013-01-22 for device and method to control an electron beam for the generation of x-ray radiation, in an x-ray tube.
This patent grant is currently assigned to Siemens Aktiengesellschaft. The grantee listed for this patent is Michael Grasruck, Andreas Schaller. Invention is credited to Michael Grasruck, Andreas Schaller.
United States Patent |
8,358,741 |
Grasruck , et al. |
January 22, 2013 |
Device and method to control an electron beam for the generation of
x-ray radiation, in an x-ray tube
Abstract
A device to control an electron beam for the generation of x-ray
radiation, has an electron emitter to generate an electron beam, to
which emitter an emitter voltage can be applied, a diaphragm, at
least two control elements associated with the diaphragm to affect
the electron beam, and switching arrangement with which at least
two different electrical voltages can be applied to the at least
two control elements. The same electrical voltage is applied to
each of the at least two control elements. Upon switching the
voltage, an electrical circuit that delays the setting of the
respective voltage at the one control element is associated with
the connection line of the one control element with the switching
arrangement to switch over the voltage. The invention moreover
concerns an operating method for the device and an x-ray tube
provided with the device.
Inventors: |
Grasruck; Michael (Erlangen,
DE), Schaller; Andreas (Erlangen, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Grasruck; Michael
Schaller; Andreas |
Erlangen
Erlangen |
N/A
N/A |
DE
DE |
|
|
Assignee: |
Siemens Aktiengesellschaft
(Munich, DE)
|
Family
ID: |
43571038 |
Appl.
No.: |
12/857,676 |
Filed: |
August 17, 2010 |
Prior Publication Data
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|
|
|
Document
Identifier |
Publication Date |
|
US 20110038460 A1 |
Feb 17, 2011 |
|
Foreign Application Priority Data
|
|
|
|
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Aug 17, 2009 [DE] |
|
|
10 2009 037 688 |
|
Current U.S.
Class: |
378/137; 378/136;
378/138; 378/113 |
Current CPC
Class: |
H01J
35/045 (20130101); H05G 1/38 (20130101); H01J
35/147 (20190501) |
Current International
Class: |
H01J
35/30 (20060101) |
Field of
Search: |
;378/16,113,136,137,138 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ho; Allen C.
Attorney, Agent or Firm: Schiff Hardin LLP
Claims
We claim as our invention:
1. A device to control an electron beam for generating x-ray
radiation, said device comprising: an electron emitter that emits
electrons to generate an electron beam, said emitter having an
emitter voltage applied thereto; a diaphragm located in a path of
said electrons emitted by said emitter that interacts with said
electrons to modulate said electron beam; at least two control
elements located at said diaphragm that interact with said
diaphragm to alter an effect of said diaphragm on said electron
beam; and a switching arrangement electrically connected to each of
said at least two control elements, said switching arrangement
being switchable between a first switching state that causes said
diaphragm to have a first effect on said electron beam, in which
said switching arrangement supplies a first voltage to each of said
at least two control elements, and a second switching state that
causes said diaphragm to have a different, second effect on said
electron beam, in which said switching arrangement supplies a
second voltage to each of said at least two control elements; and
an electrical circuit connected between said switching arrangement
and one of said at least two control elements, said electrical
circuit delaying respectively application of said first and second
voltages to said one of said at least two control elements upon
switching of said switching arrangement between said first and
second switching states.
2. A device as claimed in claim 1 wherein said diaphragm comprises
an aperture through which said electron beam passes, and wherein
said at least two control elements are located at opposite sides of
said aperture.
3. A device as claimed in claim 1 wherein said electrical circuit
comprises an ohmic resistor and a capacitor.
4. A device as claimed in claim 3 wherein said ohmic resistor is
connected between said switching arrangement and said one of said
at least two control elements via an electrical connection
line.
5. A device as claimed in claim 4 comprising a feedline that feeds
said emitter voltage to said emitter, and wherein said capacitor is
connected between said connection line and said feedline.
6. A device as claimed in claim 1 wherein said at least two control
elements are at least two control electrodes.
7. A device as claimed in claim 1 wherein said one of said control
elements is in electrical connection with said diaphragm.
8. A device as claimed in claim 1 wherein said first voltage has a
magnitude that is smaller than a magnitude of said emitter voltage,
and wherein said second voltage has a magnitude that is larger than
the magnitude of said emitter voltage.
9. An x-ray tube comprising: an evacuated housing; an anode
comprising an anode surface, at least said anode surface being
located in said evacuated housing; an electron emitter that emits
electrons to generate an electron beam that strikes said anode
surface to cause x-ray radiation to be emitted from said anode
surface, said emitter having an emitter voltage applied thereto; a
diaphragm located in a path of said electrons emitted by said
emitter that interacts with said electrons to modulate said
electron beam; at least two control elements located at said
diaphragm that interact with said diaphragm to alter an effect of
said diaphragm on said electron beam; and a switching arrangement
electrically connected to each of said at least two control
elements, said switching arrangement being switchable between a
first switching state that causes said diaphragm to have a first
effect on said electron beam, in which said switching arrangement
supplies a first voltage to each of said at least two control
elements, and a second switching state that causes said diaphragm
to have a different, second effect on said electron beam, in which
said switching arrangement supplies a second voltage to each of
said at least two control elements; and an electrical circuit
connected between said switching arrangement and one of said at
least two control elements, said electrical circuit delaying
respectively application of said first and second voltages to said
one of said at least two control elements upon switching of said
switching arrangement between said first and second switching
states.
10. A method to control an electron beam to generate x-ray
radiation, comprising the steps of: applying an emitter voltage to
an electron emitter to cause electrons to be emitted from said
electron emitter in an electron beam, said electrons striking an
anode surface to cause x-ray radiation to be emitted therefrom;
modulating said electron beam and the emission of x-ray radiation
with a diaphragm that interacts with said electrons in said
electron beam, said diaphragm having at least two control elements
associated therewith; applying respective voltages to each of said
at least two control elements and selectively switching said
respective voltages applied to said at least two control elements
to selectively block or permit passage of said electron beam beyond
said diaphragm to modulate said x-ray radiation; and after
switching said respective voltages applied to each of said at least
two control elements, delaying application of one of the respective
voltages applied to each of said at least two control elements to
one of said at least two control elements.
11. A method as claimed in claim 10 wherein application of said one
of the respective voltages applied to said one of said at least two
control elements is delayed by connecting an electrical circuit
between a switch, that switches said respective voltages applied to
each of said at least two control elements, and said one of said at
least two control elements, said electrical circuit introducing a
delay between said switch and said one of said at least two control
elements.
12. A method as claimed in claim 11 comprising providing an ohmic
resistor and a capacitor in said electrical circuit to delay said
one of the respective voltages to said one of said at least two
control elements.
13. A method as claimed in claim 12 wherein said electrical circuit
is connected in a connection line between said switch and said one
of said least two control elements, and comprising connecting said
ohmic resistor in said connection line.
14. A method as claimed in claim 12 comprising applying said
emitter voltage to said emitter via a feedline, and connecting said
capacitor between said connection line and said feedline.
15. A method as claimed in claim 10 comprising switching said at
least two control elements between a first voltage, among the
respective voltages applied to each of said at least two control
elements having a magnitude that is smaller than a magnitude of
said emitter voltage, and a second voltage, among the respective
voltages applied to each of said at least two control elements
having a magnitude that is larger than said magnitude of the
emitter voltage.
16. A method as claimed in claim 15 comprising, upon switching from
said first voltage to said second voltage with said second voltage
being delayed at said one of said control elements, initially
deflecting said electron beam at said diaphragm and thereafter
blocking said electron beam when said second voltage is completely
present at said one of said control elements.
17. A method as claimed in claim 15 wherein said electron beam
initially strikes said diaphragm upon switching from said first
voltage to said second voltage with said second voltage being
delayed at said one of said control elements, and wherein said
electron beam passes beyond said diaphragm when the first voltage
is completely present at said one of said control elements.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a device and a method to control an
electron beam emanating from an emitter of electrons for the
generation of x-ray radiation, in particular for the modulation of
x-ray radiation. The invention moreover concerns an x-ray tube
having such a device.
2. Description of the Prior Art
In the use of x-ray radiation for imaging in medical engineering,
there are various application cases in which modulation of the
x-ray radiation or the radiation power within different time
periods is desirable. For example, in x-ray computed tomography,
particularly in the acquisition of 2D x-ray projections of
measurement subjects that are not rotationally symmetrical, the
x-ray radiation is matched to the respective body cross section
that is to be exposed.
A further application case for modulation of x-ray radiation in
x-ray computed tomography is in computed tomography apparatuses
with two x-ray systems that are arranged on the rotating part of
the gantry, offset by approximately 90.degree. relative to one
another. In order to avoid x-ray scatter radiation generated by the
operation of the x-ray source of the other x-ray system from being
detected with the x-ray detector of the one x-ray system, the
emission of x-ray radiation by the x-ray source of the other x-ray
system should be suppressed during the readout of measurement data
of the x-ray detector of the one x-ray system. The modulation of
the x-ray radiation here is achieved by a temporary deactivation of
the x-ray radiation or a temporary suppression of x-ray
radiation.
The modulation of the x-ray radiation for the most part ensues by a
corresponding operation of the x-ray tube generating the x-ray
radiation, wherein the heating power of the thermal electron
emitter that is used to emit electrons is preferably varied to
generate and block the electron beam. The fastest response time of
the x-ray tube, or of the electron emitter of the x-ray tube, is
accordingly limited by the thermal inertia of the electron emitter.
A problem with the technique of varying the heating power, due to
the thermal inertia, for example with regard to the aforementioned
example pertaining to a computed tomography apparatus with two
x-ray systems is to suppress the emission of x-ray radiation by the
x-ray source of the other x-ray system during the short readout of
measurement data of the x-ray detector of the one x-ray system, and
to immediately apply x-ray radiation with the x-ray source of the
other x-ray system again after the readout.
A device to generate x-ray radiation that has a cathode electrode,
a grid electrode, a focus electrode, an anode and a voltage
splitter formed by ohmic resistors is described in US 2004/0114722
A1. The voltage splitter divides a tube voltage applied to the
anode in order to generate a focus voltage to be applied to the
focus electrode.
A device for fast dose modulation of x-ray radiation is known from
WO 2008/155715 A2, in which an electron beam for generation of
x-ray radiation which should be used to expose a subject strikes a
first region of an anode and in which the electron beam is
deflected by a deflection means toward a second region of the anode
if no subject should be exposed.
In U.S. Pat. No. 4,104,526 a device is described that has a cathode
to generate an electron beam, an anode to generate x-ray radiation
and a control screen to modulate or to suppress the electron beam.
Moreover, the device has means to detect the anode current as a
measurement unit for the current generation of x-ray radiation.
This anode current is used to control the potential difference
between the cathode and the control screen.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a device and a
method of the aforementioned type such that the generation and
suppression of the generation of x-ray radiation can ensue as
quickly as possible. Moreover, a suitable x-ray source should be
specified.
According to the invention, this object is achieved by a device to
control an electron beam for the generation of x-ray radiation,
that has an electron emitter that generates an electron beam, to
which emitter an emitter voltage can be applied; a diaphragm; at
least two control elements associated with the diaphragm to affect
the electron beam; and a switching arrangement with which at least
two different electrical voltages can be applied to the at least
two control elements. The same electrical voltage is applied to
each control element at a given point in time and, upon switching
the voltage, an electrical circuit that delays the adjustment of
the respective voltage at the one control element is associated
with the connection line of this one control element by the
switching arrangement in order to switch over the voltage.
The device functions such that the magnitude of the emitter voltage
is greater than the magnitude of the first voltage applied to the
two control element, such that the electron beam in a stationary
state strikes an x-ray target or, respectively, an anode to
generate x-ray radiation. To affect the electron beam, and thus to
modulate the x-ray radiation, in the switching process a second
voltage is applied to the control element to affect the electron
beam. This second voltage has a magnitude that is greater than that
of the emitter voltage. After the immediate switch over to the
second voltage, this appears with a delay (due to the electrical
voltage) at the one control element while it is essentially
immediately applied at the other control element. This delayed
adjustment leads to the situation that the electron beam is
initially deflected by the control element to affect the electron
beam in a first step and preferably strikes the diaphragm that the
control element is associated with. This deflection of the electron
beam takes place very quickly, such that the generation of x-ray
radiation can be interrupted correspondingly quickly. In a second
step that--like the first step--proceeds automatically due to
switching to the second voltage, or the delayed adjustment of the
second voltage at the one control element, the electron beam is
advantageously completely blocked if the adjustment of the second
voltage is also terminated at the one control element. The blocking
of the electron beam is based on the potential difference or
voltage difference between the emitter and the control element.
In order to be able to generate x-ray radiation again, the system
switches over from the second voltage to the first voltage again,
the magnitude of the first voltage being smaller than the magnitude
of the emitter voltage. In this case as well the first voltage
appears delayed--due to the electrical circuit--at the one control
element while it is essentially applied immediately at the other
control element. The electron beam is no longer blocked due to the
switching over to the first voltage. However, the electron beam is
initially, preferably deflected to the diaphragm by the delayed
adjustment of the first voltage at the one control element. If the
adjustment of the first voltage is also terminated at the one
control element, the electron beam to generate x-ray radiation
again strikes the x-ray target, i.e., the anode.
A fast control of the electron beam to generate and to suppress the
generation of x-ray radiation is thus possible with the device,
wherein in particular a fast deactivation of the generation of the
x-ray radiation can ensue by the deflection of the electron beam,
preferably onto the diaphragm. The subsequent blocking of the
electron beam, moreover, does not necessarily have to ensue as
quickly as possible. Rather, here it is important to keep the
defocusing of the electron beam--and therefore the generation of
unwanted x-ray radiation due to the uncontrolled impact of
electrons on the anode--as minimal as possible. Furthermore,
through the (advantageously complete) blocking of the electron beam
it is prevented that the diaphragm must meet high thermal
requirements, which would be the case if the electron beam were to
be deflected only onto the diaphragm for suppression of the
generation of x-ray radiation. In the device according to the
invention, by contrast, the electron beam strikes the diaphragm
only when the second voltage is present at the one control element
so that the electron beam is blocked. In a modulation of the x-ray
radiation by pulse width modulation, the energy application in the
diaphragm is always constant per cycle by the electron beam, and in
particular is also independent of the pulse width.
The emitter of electrons can be a field emitter, of a type known as
a cold cathode, or a heated emitter, and an emitter voltage is
applied to the emitter to generate an electron beam. The emitter
voltage drops between the emitter and the anode or an additional
electrode if the emitter and the emitter electrode form an electron
gun to generate an electron beam.
According to one variant of the invention, the diaphragm has an
aperture to pass the electron beam. Such an aperture diaphragm in
particular enables fast blocking of the electron beam. While the
electron beam passes unhindered through the opening of the
diaphragm and strikes an x-ray target (anode) to generate x-ray
radiation, this can be deflected quickly onto the diaphragm (and
thus be blocked) by switching of the voltage at the control
element, such that the generation of x-ray radiation is also
suppressed.
According to one embodiment of the invention, the electrical
circuit that delays the adjustment of the respective voltage at the
one control element has an ohmic resistor and a capacitor. The
ohmic resistor is advantageously connected between the switching
arrangement and the control elements in the connection line.
According to one variant of the invention, the capacitor is
connected between the connection line of the one control element
with the switching means and a feed line with which the emitter
voltage can be applied to the emitter. The magnitude of the
resistor and the value of the capacitor are to be selected
depending on, among other things, the desired time delay of the
adjustment of the voltage at the one control element.
In variants of the invention provide that the at last two control
elements associated with the diaphragm are electrodes, and each
control element is electrically connected with its associated
diaphragm, so that the voltage applied to that control element is
also applied to the diaphragm.
According to a further variant of the invention, the magnitude of
the first voltage is smaller and the magnitude of the second
voltage is larger than the magnitude of the emitter voltage, so
blocking and unblocking of the electron beam are possible.
The object of the invention as it pertains to the x-ray tube is
achieved by an x-ray tube having a device described above. The
emitter, the diaphragm and the control element of the device are
advantageously arranged together with an anode in a vacuum housing
of the x-ray tube.
The object of the invention as it pertains to the method is
achieved by a method to control an electron beam for the generation
of x-ray radiation, in which an electron beam generated as a result
of an emitter voltage applied to an emitter of electrons is
selectively conducted through an aperture of a diaphragm to an
anode, or is directed past a diaphragm to an anode, or is deflected
and blocked at the diaphragm to modulate the x-ray radiation. In a
switching process in which the same voltage is applied to at least
two control elements associated with the diaphragm to affect the
electron beam, the adjustment of the voltage at one of the control
elements is delayed. The method is preferably executed with the
device described above.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-7 show a device to control an electron beam respectively in
various operating states during a switching cycle.
FIG. 8 shows an x-ray tube having a device shown in FIGS. 1-7.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A device according to the invention to generate and control an
electron beam for the generation of x-ray radiation is shown in
FIG. 1. The device has an electron emitter 1, a diaphragm 2
arranged between the electron emitter 1 and an anode (not shown in
FIG. 1), two electrodes 3 and 4, an electrical circuit 5 and a
switching arrangement in the form of a switch 6.
In the exemplary embodiment of the invention, the diaphragm 2 is a
disc-shaped aperture diaphragm 2 with an aperture 7. In the case of
the present exemplary embodiment of the invention, the aperture
diaphragm 2 is electrically connected with the electrode 4.
In the exemplary embodiment of the invention, the electron emitter
1 (which, in the case of the present exemplary embodiment of the
invention, is a field emitter, thus an emitter that emits electrons
as a result of an electrical field) is connected with a feed line 8
to a voltage of UE=-120 kV that decreases between the electron
emitter 1 and the anode. The emitter can alternatively also be a
heated emitter.
The first electrode 3 is connected directly with the switch 6 with
a connection line 9. The second electrode 4 is likewise connected
with the switch 6 with a connection line 10. The connection line 10
is associated with the electrical circuit 5, wherein an ohmic
resistor R of the electrical circuit 5 is connected in the
connection line 10 and a capacitor C is connected between the
connection line 10 and the feed line 8.
In the case of the present exemplary embodiment of the invention,
the system can switch between the voltage U1=-119 kV and the
voltage U2=-121 kV with the switch 6. If it is switched to the
voltage U1, this is present both at the electrode 3 and at the
electrode 4. If it is switched to the voltage U2, this is likewise
present both at the electrode 3 and at the electrode 4.
In FIG. 1 the electrodes 3 and 4 are both at the voltage U1. As a
result of the emitter voltage UE (whose magnitude is greater than
the magnitude of the voltage U1), the electron emitter 1 emits
electrons that move in an electron beam 11 (passing through the
aperture 7 of the aperture diaphragm 2) in the direction of the
anode (not shown) to generate x-ray radiation. In FIG. 1 a
voltage-time diagram is additionally shown from which it is
apparent that the voltage at the electrode 3 (illustrated by the
solid line) and the voltage at the electrode 4 (illustrated by the
dashed line) are the same and in the stationary state.
In a first Step (illustrated in FIG. 2), with the switch 6 the
system switches from the voltage U1 to the voltage U2, which is
smaller than or, respectively, (in terms of its magnitude) greater
than the emitter voltage UE. While the voltage U2 is practically
immediately applied to the electrode 3 after the switching process,
its adjustment at the electrode 4 is delayed by the electrical
circuit 5, which is shown in the voltage-time diagram of FIG. 2.
For a short time this initially leads to a deflection of the
electron beam 11 on the aperture diaphragm 2, whereby the electron
beam 11 is already blocked in the desired manner for the generation
of x-ray radiation. The fast deflection of the electron beam 11
thereby prevents, due to a gradual expansion of the electron beam
that occurs otherwise, electrons from striking the anode in an
unwanted manner and that x-ray radiation (and, in fact, what is
known as extrafocal radiation) would be generated.
If the voltage U2 is delayed due to the resistance circuit but is
finally set in full at the electrode 4 and the aperture diaphragm 2
(which can be learned from the voltage-time diagrams of FIG. 3 and
FIG. 4 in chronological order), the electron beam 11 is completely
blocked as a result of the now stationary potential difference
between the electron emitter 1 and the electrodes 3 and 4 as well
as the aperture diaphragm 2, which means that no electron beam
strikes or, respectively, no electrons strike the aperture
diaphragm 2. The aperture diaphragm 2 thus must accept a certain
power only in the brief time between deflection of the electron
beam 11 and its blocking, such that no high thermal requirements
must be placed on the aperture diaphragm 2.
If x-ray radiation should be generated again, the system switches
again to the voltage U1 with the switch 6. While the voltage U1 is
practically immediately applied to the electrode 3 with the
switch-over, the setting of the voltage U1 at the electrode 4 is
delayed again as a result of the electrical circuit 5. As
illustrated in FIG. 5, the electron beam 11 begins to form again
with the switch to voltage U1 (which is smaller in terms of
magnitude), wherein the electron beam 11 initially strikes the
aperture diaphragm 2 (as shown in FIG. 6) as long as the voltage U1
is not yet completely applied to the electrode 4.
If the voltage U1 is also completely applied to the electrode 4,
the operating state shown in FIG. 7 (which corresponds to the
operating state shown in FIG. 1) results, namely that the electron
beam 11 passes through the aperture 7 of the aperture diaphragm and
strikes an anode (not shown) to generate x-ray radiation.
In the voltage-time diagrams, FIGS. 1 through 7 illustrate a
switching cycle for the modulation of the electron beam 11, wherein
the solid line shows the adjustment or the voltage curve over time
of the voltage applied to the electrode 3 and the dashed line shows
the adjustment or, respectively, the voltage curve over time of the
voltage applied to the electrode 4. For example, such a switching
cycle can have a cycle length of approximately 200 .mu.s, wherein
the time between the deflection of the electron beam and the
blocking of the electron beam is approximately 5 .mu.s. Assuming
that there would be a potential difference of 10 kV between
electron emitter 1 and aperture diaphragm 2, and an electron
current of approximately 1 ampere would flow, an average power
yield of approximately 250 watts would result for the aperture
diaphragm 2 in the 5 .mu.-seconds from the deflection of the
electron beam 11 until the blocking of the electron beam 11.
In FIG. 8 the device of FIGS. 1 through 7 is shown as part of an
x-ray tube 13 possessing an anode 12. The device and the anode 12
are arranged in a vacuum housing 14 of the x-ray tube 13. The
wiring of the x-ray tube 13, in particular the wiring of the
device, is not explicitly shown again in FIG. 8.
The preceding description of the invention is moreover to be
understood merely as an example. The field emitter 1 is thus only
schematically shown and can also be executed differently. The
electrodes 3 and 4 can be executed as flat or curved electrode
plates, in particular electrode plates curved in the shape of a
semicircle.
The diaphragm 2 does not necessarily have to be an aperture
diaphragm. The diaphragm can also be executed such that the
electron beam is directed past the diaphragm to generate x-ray
radiation and is deflected onto the diaphragm to block the electron
beam.
Although modifications and changes may be suggested by those
skilled in the art, it is the intention of the inventors to embody
within the patent warranted hereon all changes and modifications as
reasonably and properly come within the scope of their contribution
to the art.
* * * * *